1. Field of the Invention
My invention relates generally to the field of dental models and, more particularly, to a novel dowel-pin and locking assembly for a tooth die and to a method of making a dental model incorporating such an assembly.
2. Description of the Prior Art
In order to fabricate a crown or inlay on a tooth, an impression of a patient's mouth is taken, and a reproduction is made in the dental laboratory. Since the reproduction is a solid positive model of the mouth, it is necessary to isolate reproductions or dies of the individual teeth which have been prepared to receive a restoration. This isolation is accomplished by various types of dowel pins, each of which is secured to a respective die by either a first known process wherein a dowel-pin is inserted into the impression material before the dental model is fabricated, or a second known process wherein the entire solid dental model is first made and, then, holes are drilled above the individual prepared tooth areas or dies to receive individual dowel pins which are inserted and glued into the drilled holes.
In the fabrication of dental models and individual tooth dies, it is extremely important to have both an accurate reproduction of each tooth die and also of the relationship between each individual die and the adjacent model teeth. Since the natural teeth that are positioned in the dental arch are stationary, each individual die on the dental model must be accurately positioned with respect to the entire model so as to correspond to the natural teeth. If there is any movement of the die in the model, then the fabrication of a dental restoration (crown, inlay, etc.) will not be accurate, because, then, the model will not be an accurate reproduction of the natural teeth.
Furthermore, when individual crowns are to be fabricated to restore two or more natural teeth and are to be intentionally joined (soldered) on the model, any movement of the die or dies will cause the final restoration to be inaccurate since the movement of the dies creates an inaccurate reproduction of the natural teeth. Similarly, when crowns are fabricated on dies, and removable appliances are fabricated to be inserted on or into these crowns, any movement of the dies would produce an inaccurate restoration for obvious reasons.
The prior art is replete with various types of dowel pins for incorporation into each individual die. For example, there are (a) single vertical dowel pins with tapers, (b) double vertical pins and (c) single and double vertical pins with corresponding sleeves that are embedded into the stone model. In each case, the intention is to isolate each individual die, and the purpose of the single or double vertical pins is to prevent movement of the die on the master model. Since each die, with its inserted dowel-pin, rests upon the base of the dental model, and since this base has a flat surface, the only security of the die is the vertical dowel that penetrates into the base. As a result, a fulcrum or pivoting effect commonly occurs since there is nothing to prevent the vertically extending conventional dowel-pin from moving. As a result of this fulcrum effect, the die can move in five directions: bucally; lingually; mesially; distally; and also upwardly since there is also nothing available to prevent upward movement of the dowel-pin. Furthermore, often the die bottom that rests upon the flat base surface of the dental model is rough, broken or contains debris that prevents the die from properly seating on the surface of the model base. As a result, and in addition to the fulcrum effect of such a vertical dowel-pin, there is produced an inaccuracy in the relationship of the die to the model and, consequently, an inaccuracy in the fabrication of any restoration which is to be installed on the natural teeth. In addition, since dental models are not standard, and since tooth lengths are not standard, it is often necessary to have an extremely long die. Furthermore, dental models may be of different thicknesses. As a result, it should be clear that, as the length of the die increases, or as the thickness of the dental model increases, the so-called fulcrum effect of the die and dowel-pin also increases. Since there is no standard length of die or model, the conventional dowel-pin or pins do not prevent movement of the die relative to the model.
A pre-examination search of the prior art revealed many U.S. patents relating to means for positioning the dowel-pin when making a dental model according to the first process cited above, i.e., a process wherein the dowel-pin is positioned within a negative impression which is then filled with dental material or plaster which surrounds and embeds the dowel-pin, as opposed to the second process (with which my invention is associated) wherein the entire negative impression is filled with dental material or plaster to produce a positive master casting or model into which holes are selectively drilled for receiving subsequently inserted dowel-pins.
U.S. Pat. No. 2,851,728 shows a dental dowel-pin having a single hole therein for receiving an elongated, rod-like repositioning gauge supported in the base stone of a dental model; thus, there is no provision for preventing the above-mentioned fulcrum effect. Furthermore, the single rod-like gauge passes through more than one dowel-pin, a construction which has limited practical value compared to an individual locking device for each dowel pin.
U.S. Pat. Nos. 3,413,725; 3,454,256; and 3,521,354 merely disclose dowel-positioning systems, and also show the use of channel forming members located on the end of a dowel-pin to form a channel in the base stone for facilitating the removal of a selected tooth die from the stone.
U.S. Pat. No. 4,457,709 shows a coiled wire rod for holding a dowel-pin in position in a dental cavity of a tooth impression during the pouring of dental die casting material into the cavity.
U.S. Pat. No. 3,896,548 shows a dental model provided with horizontal wedges which are inserted in mating sockets spanning the parting lines between adjacent tooth dies for maintaining alignment of the tooth dies within the model.
U.S. Pat. Nos. 4,056,585 and 4,139,943 show dowel-pin constructions for use in a dental die.